1. Field of the Invention
The present invention relates to a method for the production of a single crystal, in which the single crystal crystallizing from a melt is subjected to a rotation with an alternating rotation direction. The present invention relates, in particular, to a method for the production of a single crystal by float-zone pulling without a crucible and by pulling the single crystal from a melt placed in a crucible.
2. The Prior Art
The pulling of single crystals made of semiconductor materials, for example silicon, germanium, gallium arsenide, gallium phosphide, or indium phosphide, is in most cases carried out by float-zone pulling without a crucible (floating zone crystal growth, FZ method) or by the method of pulling from a crucible (Czochralski method, CZ method).
In the FZ method, a polycrystalline stock rod is gradually melted with the aid of a radiofrequency coil. The molten material is converted into a single crystal by seeding with a monocrystalline seed crystal and by subsequent recrystallization. During the recrystallization, the diameter of the resulting single crystal is firstly conically enlarged (cone formation) until a desired final diameter is reached (rod formation). In the cone-formation phase, the single crystal is also mechanically supported, in order to relieve the load on the thin seed crystal. The basics of the FZ method are described, for example, in DE-3007377 A.
In the CZ method, a melt of the semiconductor material is prepared in a crucible, and a seed crystal is brought into contact with the melt surface and slowly lifted from the melt. A single crystal starts to grow on the bottom side of the seed crystal.
In both the CZ and FZ methods, it is customary to rotate the single crystal in a controlled way as a function of time, in order to achieve a maximally uniform and unimpaired crystal growth.
In conjunction with the FZ method, JP-2820027 has also described that the single crystal is rotated about its axis, in one sense or with an alternating rotation direction, during the FZ pulling. This alternating rotation is intended to cause effective mixing of the melt and therefore a homogeneous distribution of dopants.
Despite years of experience, there is a difficulty in achieving shape-stable growth of the single crystal, especially with respect to the production of single crystals with diameters of 100 mm or more. The single crystal is intended, as far as possible, to grow cylindrically. If the growth front erupts in the radial direction, a single crystal results which is deformed by bulges and is difficult or impossible to process into wafers. It is furthermore desirable, but not straightforward to achieve, for the resulting growth edges to be as narrow as possible and uniform on the circumference of the single crystal.